The Ars Magica game relies on a single core mechanic. Once you understand that, you understand enough to join a game. The mechanic is simple:

Roll a die

Add modifiers

Compare to the Ease Factor set by the Storyguide. If your result is below the ease factor, you failed (in some cases severe failure may result from very bad rolls). If your result is equal to the ease factor or higher, you succeed - with the degree of success determined by how much higher.

A Personality Trait check is made by adding the character's Personality Trait to a die roll. For example, the SG may require a Bravery check, implying anyone should add their Brave personality trait as a modifier.

A Reputation check allows to determine whether you have heard of someone. For example, the Storyguide might add a character's Reputation score to a die roll to determine whether anyone in the party has heard of him.

Very often, multiple modifiers are added to the roll. Perhaps the two commonest rolls are Ability checks and spell casting checks.

An Ability check is conducted by adding both an Ability and a Characteristic as modifiers. It is generally possible to add any Characteristic, according to the situation. For example, one might add the Guile + Communication to lie successfully, while Guile + Perception might be used to detect a lie.

Other modifiers may be piled on as needed. For example, a spell casting roll is often further modified by the local supernatural Aura.

Modifiers in Ars Magica are flexible. There may be many different ways to achieve the same result, so you should be willing to replace modifiers with others or apply circumstantial modifiers as needed. A person required to make a Brave Personality Trait roll, for example, might ask to use his Loyalty score instead. Detecting a lie is possible through Guile + Perception, as above, but also through Folk Ken + Perception.

In most cases the Storyguide sets the Ease Factor. Ease factors generally range from 3 for very easy tasks to 24 or more for exceedingly difficult tasks.

In some cases, an opposed check is appropriate - the Ease Factor is the result of another's roll. For example, a characetr's Attack Roll with his sword may be opposed by his opponents' Defense Roll.

For spells, the Ease Factor is the spell's Level, but with a slight complication - some types of spell casting may succeed even if they fall somewhat short of the spell level, but at the cost of Fatigue.

Abilities: Abilities range from 0 to infinity, with 10 as a general practical maximum. They are increased by experience points, with a certain Level achieved at a certain experience point total. Not all characters have all abilities.

Arts: Similar to Abilities, the Hermetic Arts are only available to magi. They progress faster than Abilities, requiring less experience to reach the same level.

Personality Traits: Every character has certain personality traits that reflect her personality. These typically range from -3 to +3. Different character have different personality traits, both in their numerical value and in their type. For example, one character may only have "Brave +1" written as his personality traits, while another may have "Brave -2, Sneaky +3, Curious +3".

If a trait check is called for and your character doesn't have this trait, the situation is handled according to the nature of the check. A person cannot practice medicine without knowing a thing about it, for example, so cannot possibly succeed in a medicine check unless he has the Medicine ability. A character without a written Brave personality trait, on the other hand, is just assumed to be "normally brave" (a modifier of zero).

Character development in Ars Magica is generally accomplished by spending time to improve, learn, practice, and so on. Each Season, a character develops by acquiring experience points, inventing spells, and so on.

Hermetic Magic is founded on the Hermetic Arts. These are comprised of Techniques and Forms. A Technique acts much like a verb, for example Creo for "I create". A Form acts much like a noun, for example Ignem for "fire". An effect is always created by joining a Technique with a Form, for example Creo + Ignem for creating fire.

Every magus has a list of known Formulaic or Ritual spells, which he has already invented for himself. You cast these spells by adding the relevant Technique and Form and Stamina to the die roll. Every spell has a Spell Level associated with it; if your roll reaches the spell level, you have cast it succesfully (lower rolls may or may not result in a succesful casting, depending on the severity of the failure).

You are not restricted to the list of known spells, however. A magus may literally attempt to cast nearly any spell, although casting an unknown spell is more difficult. Spells cast on-the-fly like this are known as Spontanous Spells.

Many creatures and characters in Ars Magica have Magic Resistance. In order to affect a character with Magic Resistance, you generally need to have your Penetration exceed his Magic Resistance score. You Penetration is equal to the amount by which your spell casting roll exceeds the spell's level, plus your Penetration Ability.

All of this means that lower-level spells are easier to cast, and have higher penetration. Higher level spells, however, are more powerful.